Reactive oxygen species (ROS) are molecules produced as by-products of cellular metabolism that are often generated by flavin dehydrogenase enzymes, including the NADPH oxidase family of proteins and others; they play a critical role in the control of the growth of human cancers as well as the body's inflammatory response to both infectious and non-infectious stimuli. It has been demonstrated that some human tumors, including colorectal and pancreatic cancers and pre-cancers, ovarian cancers, melanomas, and others express these oxidase proteins at high level, and that the ROS they produce are essential for the proliferation of these cells. NADPH oxidases are also expressed by cells that produce an immune response against tumors or other stimuli as well as cells of the cardiovascular system; when the oxidase activity of these pro-inflammatory immune or vascular cells is excessive, pathological damage to normal tissues may ensue.